A 0.55 V back-gate controlled ring VCO for ADCs in 65 nm SOTB CMOS

A voltage-controlled ring oscillator (VCO), consisting of delay cells, changes its oscillation frequency with the amplitude of an input signal in an analog-to-digital converter (ADC). The previously reported methods to control the frequency reduce the input range of the ADC because of voltage headroom required for the delay cells. This limits the achievable signal-to-noise ratio (SNR) under low supply voltages (<0.6 V). We present a 0.55-V ring VCO with back-gate control in silicon-on-thin-buried-oxide (SOTB) CMOS. The SOTB MOS transistor is very suitable for low-voltage operation, because the threshold voltage is low due to its small variability and low leakage current. The back-gate voltages of SOTB NMOS and PMOS transistors in delay cells are differentially controlled to deal with a full-scale input signal and extend the frequency tuning range. A ring VCO consisting of 30 delay cells is fabricated in a 65-nm digital SOTB CMOS process. It occupies 0.20×0.016 mm2 and oscillates at 556 MHz with a phase noise of −101.7 dBc/Hz at a 1-MHz offset and a power consumption of 0.72 mW from a 0.55-V supply. The proposed ring VCO achieves the best FoM (−158.0 dBc/Hz) among the previously reported low-voltage (<1.0 V) CMOS ring VCOs.